The invention relates to a fixing device for catalyzer particles, wherein the catalyzer particles are provided by a packed bed through which a gas can flow in the direction of gravity.
Catalyzers, in reactors for gas phase reactions, for instance, can be present in the form of particles in a packed bed through which the gas flow passes through in the direction of gravity. It is necessary hereby to fix the particles in order to prevent any undesired particle motions, since such motions might generate wear and tear occurring by abrasion or loss by fragmenting the particles into smaller fractions. This may result in a higher pressure loss in the catalyzer bed, or the particle fragments discharged along with the gas flow may affect downstream plant sections in an undesired way.
The problem is of even higher relevance under certain circumstances in gas phase reactions such as propane or butane dehydration, in which a process gas and a reaction gas have to be mixed prior to their entry into the catalyzer bed. In this regard, the smaller quantity of gas flow may be introduced into the other gas flow at high velocity, for instance, only a very short duration (<100 ms) of presence in the free space prior to entry into the catalyzer bed being tolerated in frequent cases. This fat requires, in addition to high flow velocities, a small distance between the mixing nozzles and the catalyzer surface. Any turbulence generated by the flow against the surface of the packed bed, and even locally formed eddy and backflow under certain circumstances, may lead to some catalyzer particles coming into motion in an undesired way, which may lead to said abrasion and/or particle breakage on the long run.
A number of different approaches are known to prevent such disadvantageous consequences. You may for instance place on the catalyzer bed a layer of very large particles which are mostly inert regarding the intended reaction. You may also use comparably large ceramic balls for this purpose which will fix the packed bed consisting of very small catalyzer particles in a similar way as the one achieved with the large particles. One of the problems, especially when using said procedures in which a short period of presence of the gas flow in the free space in front of the catalyzer bed is required, will be the heavily increased period of presence of the gas flow prior to entering into the catalyzer layer because of its flow through the ball bed. As local channels and free spaces inside the ball bed will hinder any further mixture of the gases, optimum mixing should already have taken place above the balls so that the flow through the balls will only increase the period of presence, but not represent the mixing track itself.
When using balls which are comparably large in relation to the catalyzer particles, one of the problems will be that said balls will often lead to irregular packing with industrial reactors, thus rendering the surface of the ball bed which faces the gas nozzles also irregular. The surface, which is now, on the basis of the large balls, substantially coarser and which has a stochastic structure, will increase its tendency to form local eddy and recirculation zones, which again means another negative influence on the period of presence of the gas mixture.
Another publicly known possibility to fix catalyzer particles in a packed bed consists in the covering of the bed with a permanently mounted perforated metal sheet. This would raise the problem of the catalyzer bed yielding and slumping during operation. Local hollow spaces could form underneath the sheet, which could by themselves generate recirculation zones and similar turbulence, with the result that particles could be set in motion thus leading to the disadvantageous consequences described in the foregoing. This effect is then reinforced by the acceleration of gas flow into and through the perforations in the perforated sheet.
It is the purpose of the invention to provide, in particular for packed beds which can be passed through in the direction of gravity, a fixing device for catalyzer particles which does not have the disadvantages described in the foregoing and which especially suffers from little specific pressure loss, which will remain low even if dirt should arise from the operating process; a fixing device which has a minimum influence on the period of presence of the gas flowing through; which ensures the reliable hold-down of the catalyzer particles even at high approach velocities; and which is flexible to a certain degree in order to adapt to bed changes.
The invention achieves this goal with the fixing device for catalyzer particles characterised in the detailed description below.
The fixing device of the invention means that, in a reactor, as it is described in the applicant's DE 103 59 744 A1, for instance, a layer of a metal braiding, consisting of single metal braiding elements which are firmly braided together is placed on the catalyzer particles bed. The particles are held down by the dead weight of said metal braiding. The fact that the uppermost particle layer is jammed within the metal braiding will furthermore prevents the particles from moving transversally.